1. Field of the Invention
The invention relates generally to neutron absorbing alloys, and in particular to an iron base alloy and an article of manufacture made from said alloy that can be processed to provide a unique combination of mechanical properties, corrosion resistance, and thermal neutron absorbability.
2. Description of the Related Art
Boron-containing stainless steels are used by the nuclear power industry for the storage, transportation, and control of radioactive materials. The suitability of this type of material in those applications is related to the increased thermal neutron absorption capability provided by the addition of boron, specifically the B10 isotope, to the base material. An example of such a material is a modified Type 304 stainless steel sold under the registered trademark MICRO-MELT® NEUTROSORB PLUS® and described and claimed in U.S. Pat. Nos. 4,891,080 and 5,017,437. Boron may be present in that material as natural boron, which contains approximately 18.3 weight percent B10 isotope (the balance being the B11 isotope), enriched boron, or a combination thereof. Applications for the known material in the nuclear power industry include wet spent fuel storage racks, baskets for spent fuel dry storage transportation casks, reactor control rods, burnable poison, and neutron shielding plates. The rising costs associated with natural gas and petroleum as well as the environmental issues surrounding the use of coal in the generation of electricity have sparked a renewed interest world-wide in the use of nuclear power to augment the use of fossil fuels to generate electric power. The waste products generated by current and future nuclear power plants will need to be stored either on-site or at regional or national repositories. The benefit provided by boron to the nuclear power industry is related to its effect of increasing a material's thermal neutron absorption cross-section. Higher boron loads or the use of alternative neutron absorbers by themselves or when coupled with boron could provide a technical and marketing advantage for many articles used in these applications.
Boron is the traditional standard bearer for neutron absorption in containment materials. Although boron has only the sixth largest thermal neutron cross-section of all naturally occurring materials, its low atomic mass makes it the second most effective alloying addition on a weight percent basis. All of the neutron absorption capabilities of boron are derived from the B10 isotope. However, B10 enriched boron is generally cost prohibitive for use in commercial alloy systems, and as such, natural boron is normally used. Boron has little or no solubility in stainless steel or nickel-based alloys. Instead, it generally forms borides that are enriched with Cr, Mo and Fe. For example, in the MICRO-MELT NEUTROSORB PLUS alloy an M2B phase forms with a composition of about 46% Cr, 40% Fe, 3.5% Mn, 1.0% Ni and 9.5% B.
Boron additions to austenitic stainless steels result in improved neutron absorption characteristics, increased hardness, yield strength and tensile strength, but reduced tensile ductility, impact toughness, and corrosion resistance. The reduced corrosion resistance results from a depletion of matrix Cr as a result of the formation of the Cr-rich M2B phase. Typically, boron-containing stainless steels have not been used as structural components in the United States because of the toughness and ductility limitations that are usually associated with the use of boron additions in conventionally processed alloys. Through the use of alloy modifications and powder metallurgy processing, MICRO-MELT NEUTROSORB PLUS alloys minimize the reductions in corrosion resistance, ductility, and impact toughness that are associated with the addition of boron to conventional Type 304 stainless steel. The MICRO-MELT® NEUTROSORB PLUS® alloys contain up to 2.25% B and are covered by ASTM A887 Grade “A”, while conventionally processed materials are covered by ASTM A887 Grade “B”. Some steel producers will not sell cast and wrought borated stainless steels with B contents higher than about 1.85%. This is related to the fact that there are significant processing issues with conventional cast and wrought borated stainless steels containing more than this amount of B. Such processing issues include cracking and tearing of the alloy material when it is mechanically hot worked. The powder metallurgy (P/M) borated stainless steels sold under the MICRO-MELT NEUTROSORB PLUS and MICRO-MELT NEUTROSORB trademarks offer the opportunity to provide customers with a higher neutron absorption capability as a result of the reduced segregation associated with P/M produced material. However, there is a limit of nominally 3.5% boron that can be added to NEUTROSORB PLUS alloys and still have a processable alloy (i.e., one that can be hot worked into plate or bar). In addition, if the amount of B can be reduced through the use of a second and stronger neutron attenuating material, then a higher B equivalency (BEq) can be obtained in P/M processed alloys.
The NEUTROSORB PLUS alloys contain enriched B10 or enriched B10 plus natural B to obtain BEq values higher than what would be attainable by using only natural B. Enriched B10 is, however, very expensive (on the order of $1600/lb), which makes using it not cost effective in many applications. Use of a non-enriched element that has a higher BEq than natural B and that is significantly less expensive than enriched B10 would be preferred.